A variety of electronic systems need to identify a precise point in time when a signal arrives at a receiver. Such systems include multilateration location determination systems, time domain reflectometry systems, and the like. When such systems calculate distances from the receiver based upon the times of arrival for electronic signals traveling at or near the speed of light, an error of as little as a few nanoseconds in identifying the precise arrival time can lead to a distance error of over a meter.
A common problem faced by these and other systems is that of distinguishing a legitimate signal from noise while simultaneously distinguishing a legitimate signal from multipath and other corrupting signals. Conventionally, the receiver generates a detection signal that is compared against a threshold. A time of arrival is indicated when the detection signal exceeds the threshold. However, the threshold must conventionally be established at a level well above a noise floor to prevent the system from falsely indicating time of arrivals in response to noise.
Unfortunately, as the threshold increases, the indicated time of arrivals become prone to errors resulting from multipath and other factors. Multipathing results when the signals reach the receiver by an indirect or reflected path, and often by two or more paths. Direct path and multipath signals reach a receiver at different times, but these different signals may coincide to some extent. In other words, a leading edge of a multipath signal may arrive soon after a leading edge of a direct path signal, and then both are present simultaneously.
Direct path and multipath signals may add to one another or subtract from one another in the receiver so that time of arrivals determined through correspondence with the threshold are inconsistent from situation to situation. If direct path and multipath signals do not interfere or add together in the receiver, then a leading edge slope of a detection signal may increase so that the detection signal actually crosses the threshold correctly. If direct path and multipath signal subtract from one another in the receiver, the detection signal may fail to reach the threshold or reach the threshold too slow.
The use of spread spectrum communication signals helps the multipath problem to some degree. Spread spectrum signals are encoded with a pseudorandom noise (PN) spreading sequence, or code. A correlator in a receiver generates a distinctive, high amplitude detection signal during a period in time while the communication signal correlates with the PN sequence. Multipath signals which arrive at the receiver after this period in time have little or no influence. However, this period may last for many tens of nanoseconds, and multipath signals arriving during this period can still corrupt the detection signal.